Various methods are known for the selective electrochemical etching of materials. According to these methods, the workpiece whose surface is to be etched is generally made the anode and contacted with the electrolyte into which the cathode is immersed. A surface can be selectively etched by limiting the contact of the electrolyte to only those areas of the surface that are to be etched. This can be achieved by covering the surface areas which are not to be etched with a suitable masking material, or by limiting the electrolyte contact to only the surface areas to be etched. The latter technique is described in German Offenlegungsschrift No. 2 707 372, and in IBM Technical Disclosure Bulletin, Volume 20, Number 7, December 1977, page 2912. Another method employed to selectively etch a surface is to locally vary the current density over the surface thereby varying the etching rate. This technique makes it possible to etch recesses with complex profiles in the etched surface. Locally different current densities can be produced by having the surface profile of the cathode correspond to the contour of indentation which is to be made in the etched surface. An article describing the details of this method is published in Scientific American, December 1974, page 30. By using this technique which employs contoured cathodes it is possible to make recesses with sides which are substantially vertical, horizontal, or inclined with respect to the original surface. A method for producing walls inclined with respect to the original surface is described in IBM Technical Disclosure Bulletin, Volume 13, Number 3, August 1970, page 726. The change in etching speed or current density which is necessary for producing the inclined plane is achieved by having the cathode designed as a plate which forms an acute angle with the surface to be etched. Another method for etching planes inclined with respect to the original surface is taught in IBM Technical Disclosure Bulletin, Volume 18, Number 4, September 1975, page 1223. Here the surface to be etched is pulled out of the electrolyte at a preselected speed. This results in the time for the etching process to vary in the pulling direction and consequently there is a variation in the quantity of the material removed as a function of distance.
The above-mentioned methods of etching planes inclined with respect to the original surfaces have the disadvantage that it is difficult to precisely and reproducibly obtain an angle of predetermined inclination. This problem is particularly true when producing very small inclinations which are required in the semiconductor devices. Inclined planes with a precisely defined angle of inclination are required in some doped semiconductors. The p- and n-conductive areas in semiconductor chips which have been doped by means of diffusion or ion implantation are characterized by a distribution of the doping atoms that vary as a function of penetration depth. The term penetration depth refers to the distance from the semiconductor surface, and the doping profile is the doping substance concentration plotted against the distance from the semiconductor surface.
Since the penetration depths are in the .mu.m range, it is obvious that it is difficult to precisely measure the penetration depth or the change in doping concentration as a function of the distance from the semiconductor surface on a vertical cut through the semiconductor. These measurements are usually made by grinding a bevelled surface onto the doped material. In this way, the penetration depth and the distances to the semiconductor surface are expanded by the factor 1/sin .alpha., the angle .alpha. being the acute angle which is formed by the bevelled plane with the semiconductor surface. It is then possible to determine the penetration depth on the bevelled plane by a staining method, and the doping profile by a stopwise measurement of the resistance along the line of steepest gradient in the bevelled plane. As the angle .alpha. is decreased, these measurements can be made more accurately. When the penetration depth is reduced, and the doped areas which are more shallow are used, the same degree of precision is ensured by reducing the angle .alpha.. As one makes smaller and smaller devices there is a need to reduce the angle to maintain the same accuracy in the measurements. The problem of making very small angles is that using existing techniques there is a rounding of the bevelled plane which results in inaccuracies in the measured penetration depths and doping profiles. For precise reproducible measuring, it is both necessary to produce a sufficiently small angle .alpha., and said angle must be made in a reproducible manner. When the inclined surfaces are ground, it is difficult to make reproducible surfaces of a uniformly high quality.
Variations in the surface quality can affect the measuring depth. To avoid the disadvantages of the grinding technique it would be of advantage to use an etching process for making the inclined planes; however, as pointed out above, the presently available etching processes are too imprecise.
Recently, semiconductor surfaces containing V-grooves have been introduced for field effect transistors (FETs). These V-grooves in field effect transistors can allow elements to be positioned in such a manner as to reduce the required space between elements which would otherwise be arranged in a parallel relationship on the semiconductor surface, i.e., to the circuit plane. With the trend toward ever increasing packing density in integrated circuits such space-saving methods are of great importance. Using existing techniques, the angle between the two legs of the V is predetermined by the orientation of specific crystal planes. This results in predetermined angle which limits the maximum depth and leg length of the groove on the semiconductor surface. If it were possible to vary the angle between the legs, the flexibility of the circuit layout would be increased considerably. With the above electrochemical etching methods of the present invention, it is possible to provide angles which will result in V-grooves having dimensions in the order of .mu.m.